In transmitting high-speed data signals having switching rates above 500 MHz over conductive lines, the effects of attenuation and inter-symbol interference impact reception much more strongly than at lower frequencies, such that special measures are needed to address these problems. Special measures may be used to improve impedance matching between the transmitter or the receiver and the conductive lines carrying the signal. Special measures may also be used to improve receiver bandwidth, such as those described in co-pending, commonly owned U.S. patent application Ser. No. 10/250,043 filed May 30, 2003.
However, use of special impedance matching and receiver measures alone may not be sufficient to permit high-speed signals on conductive lines to be properly received. The characteristics of signal channels can vary widely, despite efforts to make them uniform. For example, certain high-speed serial links are intended for communications between one board and another on rack systems having backplanes. The length and characteristics of communication paths in such systems can vary widely depending upon the positions of boards within a rack and whether the communication path goes between one rack and another. Variations become even greater in systems where transmitters having the same serial link design support a variety of selectable transmission frequencies and signaling protocols. Attenuation can vary by an order of magnitude or more in such systems. In such systems, the transmitter itself may need to provide adaptive equalization to the channel it drives.
Data transmitters are known which have adaptively equalized signal drivers. Some types of data transmitters include a finite impulse response (FIR) driver. A FIR driver is a driver which outputs data bits at amplitudes determined as a function of the values of a finite series of data bits, especially the input data bits to the driver. A FIR driver is a driver having a FIR filter function. A FIR driver typically includes a tapped delay line for performing a time domain convolution of a series of input data bits with a set of variable coefficients applied to the taps, the coefficients determining the weight of each bit of the series in a sum which determines the amplitude of the current output bit of the FIR driver. In FIR drivers, the values of the coefficients are generally fixed at the time of post-production testing, and sometimes earlier, at the time of design. Some FIR drivers are capable of modifying the values of the coefficients during operation to adaptively adjust the response of the filter to particular operating conditions. While such capability is desirable, much power and time is consumed when a FIR driver cycles through settings to settle on a set of suitable coefficient values. Therefore, cycling through settings to determine coefficient values is to be avoided, except when transmission and reception conditions change, requiring new coefficient values to be determined.
Accordingly, U.S. Pat. No. 5,519,398 issued May 21, 1996 to Satoh et al. describes a FIR filter which utilizes a “look-up table” of coefficient values stored in an eraseable programmable read only memory (EPROM). Upon powering up the FIR filter, the coefficient values are stored to the taps of the FIR filter as initial settings from the coefficient values stored in the EPROM. In this way, the FIR filter has a set of initial coefficient values which, hopefully, provide a starting point of operation for determining coefficient values appropriate for particular operational conditions under which the FIR filter is currently used. However, such look-up table can only be erased and rewritten by external intervention requiring illumination of the EPROM with ultraviolet light. The provision of a look-up table does not avoid the FIR filter from cycling through settings to determine appropriate coefficient values each time it is powered up, since the particular operating conditions vary in use. Because of that, such arrangement is undesirable for use in high-speed transmission systems such as those described above where operating conditions for each communication path vary widely.
In U.S. Pat. No. 6,266,379 B1 issued Jul. 24, 2001 to Dally, the coefficients of a FIR transmitter having a tapped delay line are stored during operation in a dynamic random access memory (DRAM) or static random access memory (SRAM). When powered off, DRAMs and SRAMs do not retain stored information. Thus, every time the transmitter is powered up, the coefficient values must be transferred from permanent storage to the DRAM or SRAM, resulting in delay for using the transmitter and/or a performance trade-off. Moreover, if the permanent storage is provided in ROM or EPROM and contains fixed initial values such as those described in the above referenced '398 Patent to Satoh et al., the FIR transmitter must again cycle through settings before reaching coefficient values appropriate to the prevailing operating conditions.
Therefore, it would be desirable to provide an adaptive data transmitter in which control information representing the values of the coefficients applied to taps of the data transmitter are updated and stored non-volatily.
It would further be desirable to operate the adaptive data transmitter upon powering up using the non-volatily stored control information.